EARLY TERTIARIES. 



157 



of terrestrial life, but discover that the fresh- water lakes, 

 the estuaries, and the seas, also teemed with many new and 

 ascending forms. It is now that we detect in these marls 

 the approximating species of our lymnece, paludinm, plau- 

 orbes, and other fresh-water shells ; the terrestrial snails, 

 helix, pupa, dausilia, &c., so slenderly represented in former 

 epochs ; and in the clays and limestones, increasing con- 

 geners of our marine gasteropods (spindle -shells, peri- 

 winkles, volutes, and cowries), fusus, cerithium, natica, 

 valuta, cyprcea all assuming so recent an aspect, that the 

 conchologist begins to rank them with living species, and 

 to reckon the chronology of strata by the percentage of ex- 

 isting shells.* It is now, too, that the seas swarm with 

 these foraminiferous organisms that attain their meridian 

 in numbers, bulk, and variety, and give rise, by their 

 myriad calcareous cases, to masses of nummulitic limestone 



, Section of do., showing its cells. 



that rival in extent and thickness the limestones of former 

 epochs, or the coral-reefs of the present day. The nummu- 

 litic limestone of the Old World, extending for thousands 

 of miles, and many hundred feet in thickness, arid the 



* The terms eocene, miocene, &c., have reference to the percentage 

 of existing shells contained in the different stages of tertiary strata, 

 thus : 



Pleistocene (most recent) from 90 to 98 of living species. 

 Pliocene (more recent) 60 to 88 



Miocene (less recent) 20 to 30 ,, 



Eocene (dawn of recent) ,, 1 to 5 



